1. Field of the Invention
The present invention relates to an object-positioning device mounted in a charged-particle beam system.
2. Description of Related Art
Mechanical vibrations produced in an analytical instrument or observation instrument using a charged-particle beam (such as an electron microscope) create one of the main factors deteriorating a performance parameter of the instrument such as spatial resolution. Such vibrations are produced by various causes, and examples of the vibrations include vibrations of a building itself, noises around the instrument, and vibrations produced by a machine adjacent to the instrument.
Various portions of the instrument are vibrated from these causes. The natural frequency of each portion of the instrument differs according to shape, material, weight, vibrational mode, and other factors.
A goniometer having a sample holder for holding and moving a sample is disclosed in FIG. 11 of JP-A-2002-124206. In this goniometer, the sample is attached to the front end of the sample holder. A control knob is coupled to the rear end of the holder. The sample holder is slidably inserted in a holder-mounting member, which, in turn, is inserted in a rotary member. The holder is held by a pressure member and screws mounted on the side surface of the rotary member.
A vibration-absorbing member in the form of a hollow disk is mounted at the joint between the control knob and the sample holder to suppress vibrations in the direction of insertion of the sample holder. The vibration-absorbing member is biased toward the control knob by a compression spring via an inertial member. When the sample holder vibrates in the direction of insertion, relative vibrations occur between the sample holder and the inertial member that is inertially at rest. At this time, the vibration-absorbing member deforms (i.e., elongates and shrinks) and converts the vibrational energy into thermal energy. Therefore, vibrations of the sample holder are suppressed.
A vibration suppressor is mounted at the rear end surface of the holder-mounting member in which a compression spring is mounted. The vibration suppressor has a rod standing upright (in the heightwise direction) from the rear end surface, a vibration-absorbing member mounted at the front-end surface of the rod, and an inertial member mounted at the outer surface of the vibration-absorbing member. That is, the vibration-absorbing member is mounted between the rod and the inertial member. If the rod vibrates in the heightwise direction of the rod, relative vibrations are produced between the rod and the inertial member that is inertially at rest. At this time, the vibration-absorbing member expands and contracts and converts the vibrational energy into thermal energy. Accordingly, vibrations of the rod are suppressed. A similar vibration suppressor is mounted on the side surface of the rotary member.
In this way, in the goniometer of JP-A-2002-124206, each of the sample holder and holder-mounting member has a separate vibration-absorbing member.
FIG. 10 of JP-A-2002-124206 shows a movable aperture device that is mounted at the side surface of the electron optical column of a microscopic analysis instrument. The diameter and position of the electron beam are controlled with the aperture. The movable aperture device has an outer spherical ball bearing whose inner surface is formed spherically, a rotatable cylinder, and a holder-mounting member inserted in the rotatable cylinder. The movable cylinder has an inner spherical bearing at its end closer to the electron optical column, the spherical bearing being formed spherically in conformity with the inner surface of the outer spherical bearing. An aperture holder is mounted at the front end of the holder-mounting member. The rotatable cylinder is supported so as to be rotatable about the center of the spherical surfaces of the outer and inner bearings by contact between the bearings.
A feed screw is coupled to the rear end of the holder-mounting member and threadedly engaged in a rotation control member. The feed screw and holder-mounting member are moved longitudinally by rotation of the rotation control member.
The rotation control member has an enlarged portion whose outer surface is covered with a vibration-absorbing member. An annular inertial member consisting of two semiannular parts interconnected is mounted on the outside of the vibration-absorbing member.
In this movable aperture device, the rotation control member, vibration-absorbing member, and inertial member together constitute a vibration absorber. The principle of vibration absorption is the same as the principle of operation of the vibration absorber of the aforementioned goniometer. When the movable aperture device vibrates around the center of rotation as if it rocks, relative vibrations occur between the rotation control member and the inertial member. Accordingly, the vibration-absorbing member interposed between them expands and shrinks. As a result, the vibrational energy is converted into thermal energy in the vibration-absorbing member. That is, oscillating motion of the vibration absorber is suppressed by this energy conversion.
As described previously, the natural vibrational frequency of each part of the instrument varies depending on shape, material, weight, vibrational mode, and other factors.
Accordingly, in the above-described prior art, each of the sample holder and devices touching the holder is equipped with a vibration suppressor according to the natural vibrational frequency. Therefore, each vibration suppressor needs to be adjusted separately. Also, the structure of the instrument is complicated.
Apart from the method described above, it is conceivable to fabricate the sample holder from a damping alloy. However, the spatial resolution of electron microscopes reaches 0.05 nm. Therefore, the amplitudes of tolerated vibrations of sample are smaller than this value. Furthermore, generally, the damping characteristics of damping alloys deteriorate steeply in a low-strain region. Accordingly, in a strain region created by vibrations smaller than atomic sizes, damping alloys may not exhibit their damping characteristics. In addition, damping alloys are expensive and increase the manufacturing cost.